Fluid pressure reduction devices are commonly used in valves. One well-known type of fluid pressure reduction device includes a stack of annular disks mounted within a fluid control valve. The stack of disks, often referred to as a "valve trim" or a "valve cage", defines a plurality of flow passages between a hollow center and an outer periphery. A valve member is movable through the hollow center to connect more or fewer of the flow passages between the valve inlet and outlet. Such devices have utilized changes in flow direction and increases in the cross-sectional area of the flow passages to reduce pressure. However, prior art fluid pressure reduction devices have not effectively applied the principles of fluid mechanics to the design of the fluid flow passages to prevent the static head from reaching the vapor pressure of the fluid, while achieving substantial pressure reduction.
A stacked valve trim which uses identical hole patterns and which creates a diagonal radial flow path is described in U.S. Pat. No. 3,529,628, issued Sep. 22, 1970 to Cummins. However, the openings in the disks were highly irregular and created between them an unpredictable resistance path. The flow paths were not designed using effective principles of fluid mechanics. Furthermore, Cummins disclosed no way to pressurize the annular area between the plug and the annular bore to avoid damage between a radial pressurized path and the next unpressurized path.
U.S. Pat. No. 4,125,129, issued Nov. 14, 1978 to Baumann also discloses a multi-plated valve trim with identical annular disks, which when angularly offset, create a vertical flow path, as shown in FIG. 14. FIGS. 18 and 19 also show identical plates which are angularly offset from each other to form a vertical flow path. However, using round holes as throttling orifices greatly reduces the flow capacity and creates a high amount of contraction due to the sharp entrances to the holes, leading to excessive pressure recovery. Similar identical plates selectively overlapped and having radial, rectangular flow slots for throttling in the horizontal direction and in one plane are disclosed in U.S. Pat. No. 4,356,843, issued Nov. 2, 1982 to Baumann.
U.S. Pat. No. 5,769,122, issued Jun. 23, 1998 to Baumann et al. discloses fluid pressure reduction devices including plates having restrictions with streamlined inlets followed by long and wide settling chambers after each contraction and providing radially outward flow paths in two planes. The configuration shown in FIGS. 10 and 11 and orifices 62 shown in FIG. 7, for example, permit a gradual decrease in velocity, corresponding to high pressure recovery, which may be suitable for gases in order to obtain supersonic velocities, but is detrimental for liquids which are prone to cavitate. The '122 patent also teaches several restrictions within one horizontal plane, allowing for unhindered pressure recovery of at least 50%.
U.S. Pat. No. 5,687,763, issued Nov. 18, 1997 to Steinke discloses a fluid flow control device having tortuous turns within radial flow paths created between openings of a pair of identical plates. The pressure drop is created through abrupt turns rather than by the use of orifices. Furthermore, there is no provision for gradual pressure reduction along the gap between the valve plug and the cage bore.
All of the known prior art fluid pressure reduction devices have exhibited one or more disadvantages, including but not limited to cavitation, insufficient pressure reduction, failure to provide for gradual pressure reduction along the gap between the valve plug and the cage bore, low flow capacity and difficulty in manufacturing. Accordingly, there is a need for new and improved fluid pressure reduction devices.